In the 150 years since Darwin proposed the theory of evolution by natural
selection, a mountain of evidence has accumulated to support the theory.
A greatly expanded fossil record since Darwin's time, the discovery of DNA and
the process of genetic replication, an understanding of radioactive decay,
observations of natural selection in the wild and in laboratories, and evidence in the
genomes of many different organisms, including humans, have all bolstered the validity
of the theory of evolution.

2. How can you know what happened millions of years ago if no one was there to see it?

Evidence and observation are the building blocks
of all scientific inquiry; evolutionary science is no different.
Evidence in the form of the fossil record, geological formations,
and genetics attest to change having taken place and give clues to
how evolution works. The theory of evolution puts these clues together into a
cohesive explanation of the diversity of living things. Like all theories, the theory of
evolution relies on tangible evidence as well as inference for those things that can't be
observed directly. It is important to remember that Earth itself contains evidence of life in the past,
and that this evidence provides critical support for the theory of evolution.

Opponents of evolution point to gaps in the fossil record as proof that the theory is invalid.
They say the fossil record fails to show what are called "transitional forms," generally the in-between
stages as one type of creature evolved into another. The fossil record certainly has gaps, mostly because
the conditions required to create fossils have been rare ever since life began on Earth. A very small percentage
of animals that have lived and died ever became fossils. Thus, many pieces of the puzzle are missing; some
will never be found. Nonetheless, we have many, many fossils that illustrate evolutionary transitions between
fish and amphibians, between reptiles and mammals, between dinosaurs and birds, and in many lineages
such as whales and horses. And new fossils continue to reveal transitional forms that some said don't exist.